90 FLIGHT International, 18 January 1973
Guiding anti-ship missiles
BUj
Avionics
EXOCET, THE FRENCH SHIP-TO-SHIP missile, which has also been bought by the Royal Navy, is
guided by inertial navigation for the
greater part of its flight, before
switching on its Electronique Marcel
Dassault radar, which locates and
locks onto the target. The radar is not
switched on until the missile is near
the end of its flight in order to give
the minimum possible reaction time
for the target's jamming system to
operate.
The initial position of the target
is determined by the launch-ship's
radar and thus its relative bearing
and range are both reasonably
accurate. Exocet's maximum range is
20 n.m., although it will probably
be more effective at shorter ranges,
say about 15 n.m. Under these condi
tions its time of flight is unlikely to
be more than 2min, so the "aim off"
to allow for the target's movement
during flight is very small. For
example, a ship steaming at 30kt
would have moved only a mile from
its original position. It follows that
the missile's radar would not need
to search a very wide, arc, but the
actual width needed depends very
much on the separation between
missile and target, and increases as
the missile approaches the target.
It has long been a dream of missile
builders and users to devise a ship-
based weapon which could be fired
well outside the radar range of the
target, thus creating greater surprise,
i.e. an OTH (over-the-horizon) missile.
With present techniques, it would
have to be fired on a target position
obtained from an aircraft, or another
ship. Such position information may
not be as accurate, relative to the
launching ship, as that obtained by
her own radar, so the OTH missile's
radar has to be capable of searching
a much wider arc. The electronic
and mechanical arrangements then
become more difficult.
The joint French (Matra) and
Italian (Oto Melara) missile Otomat
will shortly be coming into service in
the Italian Navy. It has a maximum
effective range of over 40 n.m. Its
Thomson-CSF radar is capable of
searching an arc which includes a
ship crossing at up to 40kt at what
ever the range.
Russian designers have gone for
OTH surface missiles from the earliest
days. Their standard ship missile, the
Shaddock, has a range variously
estimated as between 300 and 450 n.m.
The exact form of guidance employed
is not clear, but one possibility is that
the aircraft illuminates the target with
its radar and the missile homes on
the reflected radar waves. Thus the
missile would only be fitted with a
directional radar receiver; a process
long employed for anti-aircraft
missiles and known as active-homing
or beam-riding.
Western specialists are not enthusi
astic about mid-course guidance by
aircraft because, apart from the possi
bilities of jamming the radar, the air
craft calls attention to itself and is
very vulnerable to attack by missiles
or fighters.
That is not to say that Nato has
discarded aircraft-guidance techniques
altogether. An alternative method of
guiding an OTH missile of limited
range, say 50 n.m., might be for the
launching ship to fly a helicopter
fitted with a detection radar. The heli
copter remains in the vicinity of the
ship, but acquires the target on its
radar at high altitude. The missile
then homes on the reflected radar
energy, using a semi-active homing
system.
A variation of this theme would
be for the helicopter to pass the posi
tion of the target to the ship for
insertion into the missile's inertial-
navigation system. Provided the heli
copter remains close to the ship, and
its position can be accurately deter
mined, the location of the target can
be determined with sufficient accuracy
for the missile to use.
The US Navy is developing the
Harpoon missile, which is understood
to have an over-the-horizon perform
ance and which can be launched by
ships or aircraft.
A ship-launched OTH missile which
does not require radar guidance from
either ship or aircraft thus has an
obvious advantage. Prior illumination
by radar is an effective early-warning
to the target. Also, radars can be
jammed however they are carried or
used.
Infra-red homing equipment is much
more difficult to jam. It is passive in
nature, simply picking up informa
tion without having to transmit
signals for it. If the initial position
of the target can be determined with
out the target being aware of the
action, an IR-guided missile can be
launched with a good chance that its
flight will be undetected.
There are, of course, ways of foxing
IR-homing missiles by presenting
them with other, more "attractive"
targets, such as flares, which radiate
more intensely. The flare may be
fired from a gun to a position well
clear of the ship, or it can be dropped
over the side. In both cases the critical
factor is the time taken to deploy the
flare after the missile has been
detected.
Such techniques appear quite
realistic, if some way in the future.
The target's position might be
obtained from the bearing of its radar
transmissions, or from monitoring the
noise of its passage through the
water, using hydrophones. The missile
could then be fired by a ship well
below the horizon, or even better,
from a submarine under water.
An infra-red homing device would
need to have a wide arc of search,
or to be sufficiently sensitive to pick
up the target's IR emission at long
range, thus reducing the width of its
search pattern.
Television guidance
Television is another possible
passive guidance system and there are
two forms. In both cases a television
camera is mounted in the nose of the
missile, but in the first method its
pictures are transmitted back to the
controlling ship or aircraft, and an
operator steers the missile to hit, also
sending his commands by radio. In
the second case the television camera
measures the angle between its axis
and that of the missile, and uses this
error signal to keep the missile's
flight-path directed towards the target.
The first method is used by the
air-launched Anglo-French, anti-ship
Martel missile, but it suffers from the
disadvantage that the radio trans
missions can be intercepted and
jammed. The second method, used by
the USAF's air-ground Maverick,
would certainly give no warning to
the target, but there is always the
possibility of the missile's television
looking at the wrong target.
At present operator-guided tele
vision missiles use microwave fre
quencies for data-transmission and
guidance, implying line-of-sight ranges
only. This is satisfactory for air-
launched missiles where the aircraft
can increase its height, and thus its
horizon, at will. For surface-launched
missiles this form of guidance limits
the missile's range once again to that
of the ship's horizon. No television-
guided missile has yet been developed
for use by ships and if an OTH
weapon was required it would cer
tainly have to be of the self-guided
type.
Ship-to-ship missiles are still in their
infancy. The only navies to have them
in operational service are those of
Russia, Israel (with its Gabriel missile)
and Greece, which has just fitted
Exocet in some patrol boats. ex.